There has been hitherto known a fluid transmission device including a pump impeller coupled to a drive shaft, a turbine runner coupled to a driven shaft, a lock-up clutch for directly coupling the turbine runner to the drive shaft and a damper mechanism interposed between the turbine runner and the lock-up clutch (for example, refer to Patent Literature 1). The damper mechanism of the fluid transmission device includes an inertia mass body relatively rotatable with respect to the lock-up clutch and the turbine runner, a first elastic body connecting the inertia mass body to the lock-up clutch and a second elastic body connecting the inertia mass body to the turbine runner. The inertia mass body further includes a mass main body, an additional mass body having a relatively smaller inertia mass than the mass main body and a third elastic body. The additional mass body is relatively rotatable with respect to the mass main body and connected to the mass main body through the third elastic body, which forms a dynamic damper with the third elastic body. In the fluid transmission device, the total inertia mass of the mass main body and the additional mass body is set to 0.7 times or more of the inertia mass of the turbine runner (including an engaging means), thereby improving the damping capacity of torsional vibration in a low rotation region of the damper mechanism and expanding a lock-up area to the low rotation region.